This invention relates to a mass spectrometer, and more particularly relates to collision-induced dissociation (CID) in a tandem mass spectrometer or in an ion guide.
Radio frequency (RF) only multipole spectrometers, more particularly quadrupole spectrometers, are widely applied in mass spectrometry and nuclear physics, due to their ability to transport ions with minimal losses. During such transportation of the ions, the initial ion positions and velocities change, but the total phase space volume occupied by the ion beam remains constant (see Dawson, Quadrupole mass spectrometry and its applications). However, if a buffer gas is introduced into the ion guide, a dissipative process occurs, due to ion molecule collisions, and this enables an ion beam to be focused onto the quadrupole axis after the initial velocities have been damped.
Collisional quadrupole or other multipole devices have been used as an ion guide providing an interface between an ion source and a mass spectrometer, or alternatively as a collision cell for collision-induced dissociation (CID) experiments. As a straightforward interface, collisional damping reduces the space and velocity distributions of the ions leaving the ion source, thus improving the beam quality. For CID experiments, primary ions having relatively large velocities enter the multipole and collide with buffer gas molecules, so collision-induced dissociation takes place. The multipole helps to keep both primary ions and fragment ions, resulting from the collision-induced dissociation, close to the axis and to deliver them to the exit for further analysis. Collisions inside the multipole spectrometer again act to reduce the space and velocity distribution of the ion beam.
Ion motion in a perfect quadrupole field is governed by Mathieu""s equation (See Dawson as cited above); ions oscillate around the quadrupole axis at an appropriate fundamental frequency which is determined by their m/z and quadrupole parameters, and is independent of ion position and velocity. If the frequency of any periodic forces acting on ions coincides with the ion fundamental frequency, then resonance excitation takes place. Similar resonance excitation is widely applied in quadrupole ion trap or in ion cyclotron resonance mass spectrometers (R. E. March, R. J. Hughes, Quadrupole storage mass spectrometry, 1989, John Wiley and Sons).
These properties of spectrometers have been employed in many ways. Thus, in U.S. provisional patent application No. 60/046,926 filed May 16, 1997, there is disclosed a high pressure MS-MS system. This was intended to provide improvements to a conventional triple quadrupole mass spectrometer arrangement, employing two precision quadrupole mass spectrometers separated by an RP-only quadrupole which is operated as a gas collision cell. The first mass spectrometer is used to select a specific ion mass-to-charge ratio (m/z), and to transmit the selected ions into the RF-only quadrupole or collision cell. In the RF-only quadrupole collision cell, some or all of the parent ions are fragmented by collisions with the background gas, commonly argon or nitrogen, at a pressure of up to several millitorr. The fragment ions, along with any unfragmented parent ions are then transmitted into the second precision-quadrupole which is operated in a mass resolving mode. Usually, the mass resolving mode of this second spectrometer is set to scan over a specified mass range, or else to transmit selected ion fragments by peak hopping, i.e. by being rapidly adjusted to select specific ion m/z ratios in sequence. The ions transmitted through this spectrometer are detected by an ion detector. A problem with this conventional arrangement is that the two mass resolving quadrupoles are required to operate in the high vacuum region (less than 10xe2x88x925 torr), while the intermediate collision cell operates at a pressure up to several millitorr. That earlier invention was intended to simplify the apparatus and eliminate the necessity for separate RF-only and resolving spectrometers at the input to the apparatus. Instead, a single quadrupole is provided, operating in the RF-mode to act as a high pass filter. Additionally, this quadrupole is provided with an AC field, which can be identified as a xe2x80x9cfiltered noise fieldxe2x80x9d, which contains a notch in the frequency range corresponding to the mass of an ion of interest. This notch can be moved, to select and separate desired ions.
Other older proposals can be found, for example, in U.S. Pat. No. 5,420,425 (Bier et al. and assigned to Finnigan Corporation). This relates to an ion trap mass spectrometer, for analyzing ions. It has electrodes shaped to promote an enlarged ion occupied volume. A quadrupole field is provided to trap ions within a predetermined range of mass to charge ratios. Then, the quadrupole field is changed so that trapped ions with specific masses become unstable and leave the trapping chamber in a direction orthogonal to the central axis of the chamber. The ions leaving the spectrometer are detected, to provide a signal indicative of their mass-to-rations. One method that is taught in this patent is to first introduce ions within a predetermined range of mass-to-charge ratios into the chamber and subsequently change the field to select just some ions for further manipulation. The quadrupole field is then adjusted so as to be capable of trapping product ions of the remaining ions, and the remaining ions are then dissociated or reacted with a neutral gas to form those product ions. Subsequently, the quadrupole field is changed again, to remove, for detection, ions whose mass-to-charge ratios lie within the desired range, which ions are then detected.
The first technique taught above is complex, and requires a number of separate quadrupoles or the like, and the ability to move the ions sequentially through the different quadrupole sections. The technique taught in the Finnigan patent is complex and requires a number of steps. Also, it is concerned with ion traps and not a flow quadrupole. Accordingly, it is desirable to provide one technique which, in one device, readily enables ions of a selected mass-to-charge ratio to be subject to collision-induced-dissociation (CID) or fragmentation, so that the fragments can be transported further for subsequent analysis. It is desirable to provide this in a single device, since movement of ions from one device to another inevitably leads to some losses. Similarly, the techniques of the Finnigan patent work with pulse ion sources, but attempts to use them for continuous ion flow, for instance from an electrospray ion source, will lead to inefficiencies. In this field, spectrometers are frequently used to analyze small samples, and often, high efficiency is required, if any reliable reading or measurement is to be obtained.
A further proposal is found is published European patent application 0529885, to the assignee of the present invention. This discloses a multipole inlet system for ion traps. They both suggest the possibility of ejecting unwanted ions by resonant ejection, and also exciting ions by excitation at their lowest or other resonant frequencies sufficiently to cause collision-induced dissociation.
In accordance with a first aspect of the present invention, there is provided a method of analyzing a substance, the method comprising the steps of:
(1) ionizing the substance to generate a stream of ions;
(2) supplying the stream of ions to a quadrupole ion guide;
(3) providing a buffer gas in the ion guide;
(4) applying a radio frequency field by the quadrupole ion guide to maintain desired ions in a stable trajectory through the ion guide;
(5) in addition to the radio frequency field applied in step (4), applying a periodic change to the ion guide to cause resonance excitation of ions having a selected m/z ratio whereby the selected ions acquire increased kinetic energies resulting in enhanced collision-induced dissociation with the buffer gas;
(6) applying at least one additional excitation field in the quadrupole which additional excitation field is selected to cause resonance excitation of one of an additionally selected parent ion and a fragment ion; and
(7) analyzing the ion spectrum after fragmentation.
The selected ions preferably are subject to resonance excitation by one of: application of an additional field in the quadrupole, either by being applied to the existing rod set or by application to extra electrodes or rods provided for that purpose; amplitude modulation of the radio frequency field applied by the quadrupole; frequency modulation of the radio frequency field applied by the quadrupole; and periodic variation in the quadrupole radius, the resonance excitation being at a frequency different from the frequency of the radio frequency field.
With a buffer gas in a quadrupole there is an excitation threshold below which all energy acquired over one excitation period dissipates in collisions. So, the value of the threshold reflects the collision properties of the excited ions, and thus the ion cross-section and mobility could be measured.
A variant of this first aspect of the present invention also provides a method of analyzing a substance, the method comprising the steps of:
(1) ionizing the substance to generate a stream of ions;
(2) passing the stream of ions through a mass analyzer to select a parent ion;
(3) providing a quadruple ion guide and a buffer gas in the ion guide;
(4) applying a radio frequency field by the quadrupole ion guide to maintain desired ions in a stable trajectory through the ion guide;
(5) supplying the parent ions selected in the mass analyzer to the quadrupole ion guide with sufficient energy to cause collision-induced dissociation with the buffer gas and generation of primary fragment ions;
(6) in addition to the radio frequency field applied in step (4), applying a periodic change to the ion guide to cause resonance excitation of primary fragment ions having a selected m/z ratio whereby the selected primary fragment ions require increased kinetic energies resulting in enhanced collision-induced dissociation with the buffer gas to generate secondary fragment ions; and
(7) analyzing the ion spectrum after fragmentation.
In accordance with another aspect of the present invention, there is provided an apparatus, for analyzing a substance by resonance excitation of selected ions and selective collision-induced dissociation, the apparatus comprising:
an ion source for generating a stream of ions;
a first quadrupole ion guide, for receiving the stream of ions and mass selecting a parent ion;
a second quadrupole ion guide, for receiving the stream of parent ions and provided with a buffer gas, for collision-induced dissociation between the parent ions and the buffer gas to generate primary fragment ions;
means for generating a radio frequency signal in the second quadrupole ion guide, for guiding ions through the second quadrupole ion guide, said generating means being connected to the second quadrupole ion guide;
means for generating an excitation signal connected to the second quadrupole ion guide for causing resonance excitation of at least one the parent ions and the primary fragment ions, thereby causing collision-induced dissociation between the parent ions and the buffer gas, generating respectively primary fragment ions from the parent ions and secondary fragment ions from the primary fragment ions; and
a final mass analyzer connected to the second quadrupole ion guide, for receiving parent and fragment ions and for analyzing the ion spectrum.
A variant of this second aspect of the present invention provides an apparatus for analyzing a substance by resonance excitation of selected ions and selective collision-induced dissociation, the apparatus comprising:
an ion source for generating a stream of ions;
a quadrupole ion guide for receiving the stream of parent ions and provided with the buffer gas, for collision-induced dissociation between the ions and the buffer gas, to generate fragment ions;
means for generating a radio frequency signal in the quadrupole ion guide, for guiding ions through the quadrupole ion guide, said generating means being connected to the quadrupole ion guide;
means for generating an excitation signal connected to the quadrupole ion guide for causing resonance excitation of at least two different ions at two different frequencies, thereby causing enhanced collision-enhanced dissociation between the selected ions and the buffer gas, generating fragment ions; and
a final mass analyzer connected to the quadrupole ion guide, for receiving ions and for analyzing the ion spectrum.
While it is preferred to use a quadrupole device in the present invention, it is also envisaged that the invention could be applied to a variety of multipole instruments, such as a hexapole or octopole device. In these devices, the secular frequency of an ion depends on its position, so that the mass resolution and selectivity would not be as high. However, for some applications, the selectivity available in other multipole devices might be sufficient, and hence both the method and apparatus of the present invention could be implemented using a variety of multipole devices.